Lesson Video: Solid, liquids, and Gases Science

In this video, we learn how to identify the three common states of matter and describe and compare the properties of solids, liquids, and gases.


Video Transcript

In this video, we will learn how to identify the three common states of matter and describe and compare the properties of solids, liquids, and gases.

Say we have a bottle of perfume. There’s a simple experiment we can do. First, we’ll take the lid off and weigh the bottle of perfume. Next, we’ll leave the bottle of perfume near an open window for 24 hours. When we come back and weigh the perfume again, the mass will have decreased by quite a bit. Why did that happen? Well, everything is made of invisibly small particles that are called molecules. Molecules are the smallest unit of matter that can exist freely with well-defined physical and chemical properties. What’s going on here is that the molecules are escaping the container during that 24-hour period. As the molecules that make up the perfume leave the bottle, the mass decreases.

The perfume is a liquid, which is a state that matter can exist in. There are two other common states of matter: solid and gas. For examples of the three states, let’s consider water. Solid water is called ice. We’re likely quite familiar with liquid water. And then gaseous water is found in the air and is called water vapor. Now ice, liquid water, and water vapor all have something in common. All three are made of water molecules. So if all three are made of water molecules, why are these three states of water so different? The answer lies at the molecular level. To compare the states of matter, let’s refer to these diagrams, where each pink dot represents a molecule.

One difference between the states of matter we can directly see from the diagram. We can see that each state has different-sized spaces between the molecules. These spaces are called intermolecular spaces. We can show these intermolecular spaces exist with a simple experiment, where we mix water and ethyl alcohol. Say we have 100 cubic centimeters each of water and ethyl alcohol. When we mix them together, we’d expect that we’ll end up with 200 cubic centimeters of liquid total. But it turns out the volume of the solution is less than 200 cubic centimeters we’d expect. This is because the alcohol molecules occupy the empty space between the water molecules, or the intermolecular spaces, when we mix the two substances together.

We can see from the diagrams that these intermolecular spaces tend to be different from each state of matter. Solids are tightly packed, with the smallest intermolecular spaces. In gases, the molecules are far apart, and the intermolecular spaces are very large. Another difference between the states of matter is the movement of molecules. Molecules are constantly in motion. They can move around in space, vibrate, and rotate. In solids, the molecules are so tightly packed that they cannot really move in space; the molecules only vibrate. This is why solids are so rigid compared to liquids and gases. In a liquid, molecules can slide past each other, which is why liquids are pourable. In a gas, molecules tend to move more freely than the molecules of a liquid or solid substance, and the molecules are fast moving.

Now, why do we see these trends in molecular spaces and motion for the states of matter? Well, it all has to do with the forces of attraction between molecules. Forces of attraction are called intermolecular forces. Solids are held together so tightly because the intermolecular forces between solid particles are very strong. Liquids have intermolecular forces that are much weaker than solids. And gases have the weakest forces of attraction between particles. This should make sense to us. After all, it’s much easier to cleanly divide a liquid in half than it is to divide a solid. This is because the strong intermolecular forces in the solid keep the particles together.

Now the differences in intermolecular forces means that solids, liquids, and gases have different properties. The properties we’ll discuss are the shape and volume of the different states. Solids have a definite volume and a definite shape. This means that if you put a solid in a different container, the volume and shape stay the same. Liquids have a definite volume but no definite shape. This means if we put a liquid in a different container, the volume will stay the same but the liquid will take the shape of the bottom of the container. Finally, gases have no definite shape or volume. So a gas will completely fill a container, no matter the size.

Now that we’ve talked about solids, liquids, and gases in detail, let’s work through some practice problems.

What is the correct order, from strongest to weakest, for the attractive forces in solids, liquids, and gases? (A) Gases, solids, liquids. (B) Solids, liquids, gases. (C) Solids, gases, liquids. (D) Gases, liquids, solids. (E) Liquids, solids, gases.

Solid, liquid, and gas are the three common states that matter can be in. We’ve been asked about the attractive forces in these three states. These attractive forces are called intermolecular forces. Intermolecular forces are the forces of attraction between particles. If intermolecular forces between particles are strong, the particles are held together tightly, and they can’t move as well. If attractive forces are weak, particles aren’t held together tightly and they drift further apart.

Let’s compare these diagrams of the solid, liquid, and gas states to see if we can determine which one has the stronger intermolecular forces. In a solid, the particles are tightly packed, but in a gas the particles are able to drift away from each other. So, we would expect solids to have stronger intermolecular forces than liquids and gases do.

This question asked us to order the attractive forces in solids, liquids, and gases from strongest to weakest. Solids have the stronger intermolecular forces, and gases have the weakest. So answer choice (B), solids, liquids, and gases, is the correct answer.

The diagram below shows three different arrangements of particles. Which arrangement would you expect a solid to have?

Solid is one of the three common states of matter, the others being liquid and gas. In all states of matter, particles are in constant motion. The possible motions of these particles is translation, vibration, and rotation. The forces of attraction between particles are different for the different states of matter. Solids have strong forces of attraction between particles. The strong attraction between particles pull the particles closer together, making solids more tightly packed than liquids and gases. This means that particles in a solid can’t move around very much in space; they really only vibrate in place.

The diagram that matches the arrangement of particles we would expect a solid to have, given this discussion, is diagram (A). In diagram (A), the particles are tightly packed together like solid particles are. So we would expect the diagram that shows a solid to be diagram (A).

A sample of pale-green gas is added to the odd-shaped container shown below and then sealed. Which image shows how the gas will occupy the container?

This question asked us about gases, which are one of the three common states of matter. The states of matter have different properties. Two that are relevant for this question are the shape and volume of the states of matter. Solids have a definite shape and a definite volume. This means that they keep their shape and size if we move the solid to another container. Liquids take the shape of any container we put them in, so they have no definite shape. But the volume of the liquid will stay the same. Gases, on the other hand, have no definite shape or volume. Gases will completely fill any container that they occupy because of this, no matter the shape or size of the container.

We’ve been asked to choose the image that shows what will happen when the pale-green gas occupies the odd-shaped container. We know the gas will completely occupy the container they’re in because they have no definite shape or volume. The answer choice that shows the gas completely occupying the container is answer choice (D), which is the correct answer to this question.

With that, we’ve reached the end of the video. So let’s summarize what we learned today. The three common states of matter are solid, liquid, and gas. Solids have the strongest forces of attraction between molecules, while gases have the weakest. Intermolecular spaces are the spaces between molecules. Solids are tightly packed with small intermolecular spaces. Liquids have larger intermolecular spaces. Gases have larger still intermolecular spaces, with molecules spread apart.

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